Editing Simple Mail Transfer Protocol (section)

==History==
{{Further|History of email}}
=== Predecessors to SMTP ===
Various forms of one-to-one [[Instant messaging|electronic messaging]] were used in the 1960s. Users communicated using systems developed for specific [[mainframe computer]]s. As more computers were interconnected, especially in the U.S. Government's [[ARPANET]], standards were developed to permit exchange of messages between different operating systems. 

Mail on the ARPANET traces its roots to 1971: the Mail Box Protocol, which was not implemented,<ref>[http://www.multicians.org/thvv/mail-history.html The History of Electronic Mail] {{Webarchive|url=https://web.archive.org/web/20171202025034/http://www.multicians.org/thvv/mail-history.html |date=December 2, 2017 }}'', [[Tom Van Vleck]]: "''It is not clear this protocol was ever implemented''"''</ref> but is discussed in {{IETF RFC|196}}; and the [[SNDMSG]] program, which [[Ray Tomlinson]] of [[BBN Technologies|BBN]] adapted that year to send messages across two computers on the ARPANET.<ref>[//openmap.bbn.com/~tomlinso/ray/firstemailframe.html ''The First Network Email''], [[Ray Tomlinson]], BBN</ref><ref>Picture of "[//kingsmtp.com/the-first-email-computer/ The First Email Computer]" by Dan Murphy, a [[PDP-10]]</ref><ref>[http://www.opost.com/dlm/tenex/ Dan Murphy's TENEX and TOPS-20 Papers] {{webarchive |url=https://web.archive.org/web/20071118204016/http://www.opost.com/dlm/tenex/ |date=November 18, 2007 }}</ref> A further proposal for a Mail Protocol was made in RFC 524 in June 1973,<ref>{{IETF RFC|524}} – A Proposed Mail Protocol</ref> which was not implemented.<ref>{{Cite journal |last=Crocker |first=David H. |date=December 1977 |title=Framework and Functions of the "MS" Personal Message System |url=https://www.rand.org/content/dam/rand/pubs/reports/2007/R2134.pdf |journal=The RAND Corporation |access-date=April 17, 2022 |archive-date=May 13, 2022 |archive-url=https://web.archive.org/web/20220513083616/https://www.rand.org/content/dam/rand/pubs/reports/2007/R2134.pdf |url-status=live }}</ref>

The use of the [[File Transfer Protocol]] (FTP) for "network mail" on the ARPANET was proposed in RFC 469 in March 1973.<ref>{{IETF RFC|469}} – Network Mail Meeting Summary</ref> Through RFC 561, RFC 680, RFC 724, and finally RFC 733 in November 1977, a standardized framework for "electronic mail" using FTP mail servers on was developed.<ref name=":12">RFC 733, 21 November 1977, Standard for the Format of ARPA Network Text Message</ref><ref>{{Cite news |date=2023-05-20 |title=A history of e-mail: Collaboration, innovation and the birth of a system |url=https://www.washingtonpost.com/national/on-innovations/a-history-of-e-mail-collaboration-innovation-and-the-birth-of-a-system/2012/03/19/gIQAOeFEPS_story.html |access-date=2024-07-07 |work=Washington Post |language=en-US |issn=0190-8286}}</ref>

SMTP grew out of these standards developed during the 1970s. Ray Tomlinson discussed network mail among the [[International Network Working Group]] in ''INWG Protocol note 2'', written in September 1974.<ref name=":03">{{Cite journal |last=McKenzie |first=Alexander |date=2011 |title=INWG and the Conception of the Internet: An Eyewitness Account |journal=IEEE Annals of the History of Computing |volume=33 |issue=1 |pages=66–71 |doi=10.1109/MAHC.2011.9 |issn=1934-1547 |s2cid=206443072}}</ref> INWG discussed protocols for electronic mail in 1979,<ref>Barber, D., and J. Laws, "A Basic Mail Scheme for EIN," INWG 192, February 1979.</ref> which was referenced by [[Jon Postel]] in his early work on Internet email. Postel first proposed an Internet Message Protocol in 1979 as part of the [[Internet Experiment Note]] (IEN) series.<ref>{{Cite IETF|ien=85}}</ref><ref>{{Cite IETF|ien=113}}</ref><ref>{{Cite web |title=Internet Experiment Note Index |url=https://www.rfc-editor.org/ien/ien-index.html |access-date=2024-07-07 |website=www.rfc-editor.org}}</ref>

=== Original SMTP ===
In 1980, Postel and Suzanne Sluizer published {{IETF RFC|772}} which proposed the Mail Transfer Protocol as a replacement for the use of the FTP for mail. {{IETF RFC|780}} of May 1981 removed all references to FTP and allocated port 57 for [[Transmission Control Protocol|TCP]] and [[User Datagram Protocol|UDP]],<ref name=jp>{{Cite report |url=https://doi.org/10.17487/rfc0821 |title=Simple Mail Transfer Protocol |last=Postel |first=J. |date=August 1982 |publisher=RFC Editor}}</ref> an allocation that has since been removed by [[Internet Assigned Numbers Authority|IANA]]. In November 1981, Postel published {{IETF RFC|788}} "Simple Mail Transfer Protocol".

The SMTP standard was developed around the same time as [[Usenet]], a one-to-many communication network with some similarities.<ref name=jp/>

SMTP became widely used in the early 1980s. At the time, it was a complement to the [[UUCP|Unix to Unix Copy Program]] (UUCP), which was better suited for handling email transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time. Both used a [[store and forward]] mechanism and are examples of [[push technology]]. Though Usenet's [[Usenet newsgroup|newsgroups]] were still propagated with UUCP between servers,<ref>{{cite web| url = http://tldp.org/HOWTO/Usenet-News-HOWTO/x64.html| title = Tldp.org| access-date = August 25, 2007| archive-date = August 17, 2007| archive-url = https://web.archive.org/web/20070817090558/http://tldp.org/HOWTO/Usenet-News-HOWTO/x64.html| url-status = live}}</ref> UUCP as a mail transport has virtually disappeared<ref>{{cite web| url = https://tools.ietf.org/html/draft-barber-uucp-project-conclusion-05| title = draft-barber-uucp-project-conclusion-05 – The Conclusion of the UUCP Mapping Project<!-- Bot generated title -->| date = December 19, 2000| access-date = August 25, 2007| archive-date = October 13, 2007| archive-url = https://web.archive.org/web/20071013094756/http://tools.ietf.org/html/draft-barber-uucp-project-conclusion-05| url-status = live| last1 = Barber| first1 = Stan O.}}</ref> along with the "[[bang path]]s" it used as message routing headers.<ref>The article about [[Sender Rewriting Scheme|sender rewriting]] contains technical background info about the early SMTP history and source routing before {{IETF RFC|1123}}.</ref>

[[Sendmail]], released with [[Berkeley Software Distribution|4.1cBSD]] in 1983, was one of the first mail transfer agents ([[Message Transfer Agent|MTA]]) to implement SMTP.<ref>{{Citation| author = Eric Allman| year = 1983| title = Sendmail – An Internetwork Mail Router| series = BSD UNIX documentation set| publisher = University of California| location = Berkeley| url = https://docs.freebsd.org/44doc/smm/09.sendmail/paper.pdf| access-date = June 29, 2012| archive-date = May 20, 2013| archive-url = https://web.archive.org/web/20130520171455/http://docs.freebsd.org/44doc/smm/09.sendmail/paper.pdf| url-status = live}}</ref> Over time, as BSD Unix became the most popular operating system on the Internet, Sendmail became the most common mail transfer agent.<ref>{{Citation |author=Craig Partridge |year=2008 |title=The Technical Development of Internet Email |volume=30 |issue=2 |pages=3–29 |publisher=IEEE Computer Society |series=IEEE Annals of the History of Computing |doi=10.1109/MAHC.2008.32 |s2cid=206442868 |url=http://www.ir.bbn.com/~craig/email.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110512165437/http://www.ir.bbn.com/~craig/email.pdf |archive-date=May 12, 2011 |df=mdy-all }}</ref>

The original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial [[man-in-the-middle attack]], [[Email spoofing|spoofing]], and [[Email spam|spamming]], and requiring any binary data to be encoded to readable text before transmission. Due to absence of a proper authentication mechanism, by design every SMTP server was an [[open mail relay]]. The [[Internet Mail Consortium]] (IMC) reported that 55% of mail servers were open relays in 1998,<ref>{{cite web|author=Paul Hoffman|date=February 1, 1998|title=Allowing Relaying in SMTP: A Survey|url=http://www.imc.org/imcr-006.html|accessdate=2010-05-30|publisher=[[Internet Mail Consortium]]|archive-date=March 5, 2016|archive-url=https://web.archive.org/web/20160305105916/https://www.imc.org/imcr-006.html|url-status=live}}</ref> but less than 1% in 2002.<ref>{{cite web|author=Paul Hoffman|date=August 2002|title=Allowing Relaying in SMTP: A Series of Surveys|url=http://www.imc.org/ube-relay.html|url-status=dead|archiveurl=https://web.archive.org/web/20070118121843/http://www.imc.org/ube-relay.html|archivedate=2007-01-18|accessdate=2010-05-30|publisher=[[Internet Mail Consortium]]}}</ref> Because of spam concerns most email providers [[blocklist]] open relays,<ref>{{Cite web|date=2007-06-17|title=In Unix, what is an open mail relay? - Knowledge Base|url=http://kb.iu.edu/data/aivh.html|access-date=2021-03-15|archive-url=https://web.archive.org/web/20070617083024/http://kb.iu.edu/data/aivh.html|archive-date=June 17, 2007}}</ref> making original SMTP essentially impractical for general use on the Internet.

=== Modern SMTP ===
In November 1995, {{IETF RFC|1869}} defined Extended Simple Mail Transfer Protocol (ESMTP), which established a general structure for all existing and future extensions which aimed to add-in the features missing from the original SMTP. ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified and servers can indicate supported extensions. 

Message submission ({{IETF RFC|2476}}) and [[SMTP Authentication|SMTP-AUTH]] ({{IETF RFC|2554}}) were introduced in 1998 and 1999, both describing new trends in email delivery. Originally, SMTP servers were typically internal to an organization, receiving mail for the organization ''from the outside'', and relaying messages from the organization ''to the outside''. But as time went on, SMTP servers (mail transfer agents), in practice, were expanding their roles to become [[Mail submission agent|message submission agents]] for [[Email client|mail user agents]], some of which were now relaying mail ''from the outside'' of an organization. (e.g. a company executive wishes to send email while on a trip using the corporate SMTP server.) This issue, a consequence of the rapid expansion and popularity of the [[World Wide Web]], meant that SMTP had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as relaying of unsolicited email ([[Email spam|spam]]). Work on message submission ({{IETF RFC|2476}}) was originally started because popular mail servers would often rewrite mail in an attempt to fix problems in it, for example, adding a domain name to an unqualified address. This behavior is helpful when the message being fixed is an initial submission, but dangerous and harmful when the message originated elsewhere and is being relayed. Cleanly separating mail into submission and relay was seen as a way to permit and encourage rewriting submissions while prohibiting rewriting relay. As spam became more prevalent, it was also seen as a way to provide authorization for mail being sent out from an organization, as well as traceability. This separation of relay and submission quickly became a foundation for modern email security practices.

As this protocol started out purely [[ASCII]] text-based, it did not deal well with binary files, or characters in many non-English languages. Standards such as Multipurpose Internet Mail Extensions ([[MIME]]) were developed to encode binary files for transfer through SMTP. Mail transfer agents (MTAs) developed after [[Sendmail]] also tended to be implemented [[8-bit clean]], so that the alternate "just send eight" strategy could be used to transmit arbitrary text data (in any 8-bit ASCII-like character encoding) via SMTP. [[Mojibake]] was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only [[ASCII]]. 8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting some binary files to be transmitted almost as easily as plain text (limits on line length and permitted octet values still apply, so that MIME encoding is needed for most non-text data and some text formats). In 2012, the <code>SMTPUTF8</code> extension was created to support [[UTF-8]] text, allowing international content and addresses in non-[[Latin script|Latin]] scripts like [[Cyrillic script|Cyrillic]] or [[Chinese characters|Chinese]].

Many people contributed to the core SMTP specifications, among them [[Jon Postel]], [[Eric Allman]], Dave Crocker, [[Ned Freed]], Randall Gellens, [[John Klensin]], and [[Keith Moore]].